Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop on demand ink jet.
Great Britain Patent No. 2,007,162, which issued to Endo et al. in 1979, discloses an electrothermal drop on demand ink jet printer which applies a power pulse to an electrothermal heater which is in thermal contact with water based ink in a nozzle. A small quantity of ink rapidly evaporates, forming a bubble which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan). U.S. Pat. No. 4,490,728, which issued to Vaught et al. in 1982, discloses an electrothermal drop ejection system which also operates by bubble formation to eject drops in a direction normal to the plane of the heater substrate. Rapid bubble formation provides the momentum for drop ejection.
Commonly assigned U.S. Pat. No. 5,880,759 which issued to Kia Silverbrook on Mar. 9, 1999, discloses a drop on demand liquid printing system wherein drop ejection is effected by selective actuation of a heater acting on the meniscus (the ink-air interface) of ink to be ejected. For this class of printer, the heater element may take the form of a ring or a part of a ring at the top surface of the print head. The top surface through which the orifices open generally defines an "orifice plane." The placement accuracy of ejected drops is influenced by the line of contact between the meniscus of the ink to be ejected and the top surface of the print head. If the contact line between the ink and the orifice surface is not symmetrically disposed about the orifice, the drops will not necessarily be ejected in a desired direction perpendicular to the orifice plane.
Continuous ink jet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell. Conventional continuous ink jet utilizes electrostatic charging tunnels that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a "catcher") may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium. U.S. Pat. No. 3,878,519, which issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
In another class of continuous ink jet printers, such as disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/954,317 entitled CONTINUOUS INK JET PRINTER WITH ASYMMETRIC HEATING DROP DEFLECTION filed in the names of Chwalek, Jeanmaire, and Anagnostopoulos on Oct. 17, 1997, an ink jet printer includes a delivery channel for pressurized ink to establish a continuous flow of ink in a stream flowing from a nozzle bore in a direction of propagation related to the orifice plane. A heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter causes the stream to break up into a plurality of droplets at a position spaced from the heater. Actuation of the heater section produces an asymmetric application of heat to the stream to control the direction of the stream between a print direction and a non-print direction. The placement accuracy of ejected drops is influenced by the line of contact between the meniscus of the ink to be ejected and the surface of the orifice from which the drops are ejected.
For drop ejection apparatus in which a heater acts on the ink-air interface of ink to be ejected, the need to contact the heater electrically has made it difficult to provide a heater having sufficient symmetry to ensure that drops will be ejected in a direction perpendicular to the orifice plane. An electrical heater surrounds a central bore on a substrate. Electrical leads contact the heater so that the heater can be selectively operated.
In conventional ink jet technology such as thermal ink jet, the ink meniscus remains in the nozzle bore until ejected. However, in ink jet print heads wherein the ink meniscus extends above the nozzle bore and actually spreads along the surface of the print head, it has been conventional thought that there had to be a sharp physical edge, such as a surface height step, upon which the ink meniscus could "pin" so as to keep the meniscus from expanding outwardly along the upper surface of the print head. Much inventive effort has been devoted to the manufacture of structures to pin the ink meniscus. Unfortunately, the configuration that provided a hard edge was difficult to manufacture and hard to clean.